1. Field of the Invention
The present invention relates to switch driver circuitry for use, for example, in digital-to-analog converters.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows parts of a previously-considered current-switched digital-to-analog converter (DAC) 1. The DAC 1 is designed to convert an n-bit digital input word into a corresponding analog output signal.
The DAC 1 includes a plurality of individual binary-weighted current sources 21 to 2n corresponding respectively to the n bits of the digital input word applied to the DAC. Each current source passes a substantially constant current, the current values passed by the different current sources being binary-weighted such that the current source 21 corresponding to a least-significant-bit of the digital input word passes a current I, the current source 22 corresponding to the next-least-significant-bit of the digital input word passes a current 2I, and so on for each successive current source of the converter.
The DAC 1 further includes a plurality of differential switching circuits 41 to 4n corresponding respectively to the n current sources 21 to 2n. Each differential switching circuit 4 is connected to its corresponding current source 2 and switches the current produced by the current source either to a first terminal connected to a first connection line A of the converter or a second terminal connected to a second connection line B of the converter. The differential switching circuit receives one bit of the digital input word (for example the differential switching circuit 41 receives the least-significant-bit of the input word) and selects either its first terminal or its second terminal in accordance with the value of the bit concerned. A first output current IA of the DAC is the sum of the respective currents delivered to the differential-switching-circuit first terminals, and a second output current IB of the DAC is the sum of the respective currents delivered to the differential-switching-circuit second terminals. The analog output signal is the voltage difference VA-VB between a voltage VA produced by sinking the first output current IA of the DAC 1 into a resistance R and a voltage VB produced by sinking the second output current IB of the converter into another resistance R.
FIG. 2 shows a previously-considered form of differential switching circuit suitable for use in a digital-to-analog-converter such as the FIG. 1 converter.
This differential switching circuit 4 comprises first and second PMOS field effect transistors (FETs) S1 and S2. The respective sources of the transistors S1 and S2 are connected to a common node TAIL to which a corresponding current source (21 to 2n in FIG. 1) is connected. The respective drains of the transistors S1 and S2 are connected to respective first and second output nodes OUTA and OUTB of the circuit which correspond respectively to the first and second terminals of each of the FIG. 1 differential switching circuits.
Each transistor S1 and S2 has a corresponding driver circuit 61 or 62 connected to its gate. Complementary input signals IN and INB are applied respectively to the inputs of the driver circuits 61 and 62. Each driver circuit buffers and inverts its received input signal IN or INB to produce a switching signal SW1 or SW2 for its associated transistor S1 or S2 such that, in the steady-state condition, one of the transistors S1 and S2 is on and the other is off. For example, as indicated in FIG. 2 itself, when the input signal IN has the high level (H) and the input signal INB has the low level (L), the switching signal SW1 (gate drive voltage) for the transistor S1 is at the low level L, causing that transistor to be ON, whereas the switching signal SW2 (gate drive voltage) for the transistor S2 is at the high level H, causing that transistor to be OFF. Thus, in this condition, all of the input current flowing into the common node TAIL is passed to the output node OUTA and no current passes to the output node OUTB.
When it is desired to change the state of the circuit 4 of FIG. 2 so that the transistor S1 is OFF and the transistor S2 is ON, complementary changes are made simultaneously in the input signals IN and INB such that the input signal IN changes from H to L at the same time as the input signal INB changes from L to H. As a result of these complementary changes, it is expected that the transistors S1 and S2 will switch symmetrically, that is that the transistor S1 will turn OFF at exactly the same moment that the transistor S2 turns ON. However, in practice there is inevitably some asymmetry in the turn-ON and turn-OFF speeds. This can result in a momentary glitch at the common node TAIL which may in turn cause glitches at one or both output nodes of the circuit, producing a momentary error in the DAC analog output value until all of the switches have switched completely. These glitches in the analog output signal may be code-dependent and result in harmonic distortion or even non-harmonic spurs in the output spectrum.
As the size of the glitch associated with the switching of the differential switching circuit is dependent on the symmetry of the complementary changes in the input signals IN and INB, much attention has been directed to generating and delivering these input signals to the differential switching circuit synchronously with one another. However, it is found in practice that, even if the input signals are perfectly symmetrical, the drive circuits 61 and 62 which derive the switching signals from the input signals inevitably introduce asymmetry into the switching signals SW1 and SW2 which actually control the transistors S1 and S2. Such asymmetry results in transient output current distortion in any individual differential switch circuit. Furthermore, in a DAC employing multiple differential switch circuits, it also results in a variation between the switching times of the different circuits. These variations lower the spurious-free dynamic range (SFDR) of the DAC (a measure of the difference, in dB, between the rms amplitude of the output signal and the peak spurious signal over the specified bandwidth). These variations also lead to code-dependency of the analog output signal of the converter.
According to a first aspect of the present invention there is provided switch driver circuitry comprising: first and second output nodes; a current-voltage converter connected to said first and second output nodes to provide a current path through which current is permitted to flow in a first direction from said first to said second output node, or in a second direction from said second to said first output node, when the circuitry is in use, for producing a potential difference between said first and second output nodes that is dependent upon the magnitude and direction of the current flow; and switching circuitry connected with said first and second output nodes and switchable, in dependence upon an applied control signal, from a first state, in which a current of preselected magnitude is caused to flow in said first direction through said current path, to a second state in which a current of substantially the same magnitude as said preselected magnitude is caused to flow in said second direction through said current path, a current-voltage characteristic of the current-voltage converter being such that said potential differences produced respectively in said first and second states have substantially the same magnitudes but opposite polarities.
Such switch driver circuitry can provide improved symmetry of operation.
According to a second aspect of the present invention there is provided a switch circuit comprising: first and second output nodes; a current-voltage converter connected to said first and second output nodes to provide a current path through which current is permitted to flow in a first direction from said first to said second output node, or in a second direction from said second to said first output node, when the circuitry is in use, for producing a potential difference between said first and second output nodes that is dependent upon the magnitude and direction of the current flow; switching circuitry connected with said first and second output nodes and switchable, in dependence upon an applied control signal, from a first state, in which a current of preselected magnitude is caused to flow in said first direction through said current path, to a second state in which a current of substantially the same magnitude as said preselected magnitude is caused to flow in said second direction through said current path, a current-voltage characteristic of the current-voltage converter being such that said potential differences produced respectively in said first and second states have substantially the same magnitudes but opposite polarities; a first switch element having a control terminal connected to said first output node and switchable from an OFF state to an ON state by the change in the first-output-node potential brought about when said switching circuitry is switched from one of said first and second states to the other of those states; and a second switch element having a control terminal connected to said second output node and switchable from an ON state to a OFF state by the change in the second-output-node potential brought about when said switching circuitry is switched from said one state to said other state.
According to a third aspect of the present invention there is provided a digital-to-analog converter comprising switch driver circuitry comprising: first and second output nodes; a current-voltage converter connected to said first and second output nodes to provide a current path through which current is permitted to flow in a first direction from said first to said second output node, or in a second direction from said second to said first output node, when the circuitry is in use, for producing a potential difference between said first and second output nodes that is dependent upon the magnitude and direction of the current flow; switching circuitry connected with said first and second output nodes and switchable, in dependence upon an applied control signal, from a first state, in which a current of preselected magnitude is caused to flow in said first direction through said current path, to a second state in which a current of substantially the same magnitude as said preselected magnitude is caused to flow in said second direction through said current path, a current-voltage characteristic of the current-voltage converter being such that said potential differences produced respectively in said first and second states have substantially the same magnitudes but opposite polarities; the digital-to-analog converter further comprising: a first switch element having a control terminal connected to said first output node and switchable from an OFF state to an ON state by the change in the first-output-node potential brought about when said switching circuitry is switched from one of said first and second states to the other of those states; a second switch element having a control terminal connected to said second output node and switchable from an ON state to a OFF state by the change in the second-output-node potential brought about when said switching circuitry is switched from said one state to said other state, said first switch element being connected between first and second converter nodes and said second switch element being connected between said first node and a third converter node; and a current source or current sink connected operatively to said first converter node for causing a substantially constant current to pass through said first converter node when the converter is in use.
According to a fourth aspect of the present invention there is provided a digital-to-analog converter comprising: a plurality of differential switching circuits, each differential switching circuit having switch driver circuitry comprising: first and second output nodes; a current-voltage converter connected to said first and second output nodes to provide a current path through which current is permitted to flow in a first direction from said first to said second output node, or in a second direction from said second to said first output node, when the circuitry is in use, for producing a potential difference between said first and second output nodes that is dependent upon the magnitude and direction of the current flow; switching circuitry connected with said first and second output nodes and switchable, in dependence upon an applied control signal, from a first state, in which a current of preselected magnitude is caused to flow in said first direction through said current path, to a second state in which a current of substantially the same magnitude as said preselected magnitude is caused to flow in said second direction through said current path, a current-voltage characteristic of the current-voltage converter being such that said potential differences produced respectively in said first and second states have substantially the same magnitudes but opposite polarities; each said differential switching circuit further having: a first switch element having a control terminal connected to said first output node and switchable from an OFF state to an ON state by the change in the first-output-node potential brought about when said switching circuitry is switched from one of said first and second states to the other of those states; a second switch element having a control terminal connected to said second output node and switchable from an ON state to a OFF state by the change in the second-output-node potential brought about when said switching circuitry is switched from said one state to said other state, said first switch element being connected between first and second nodes of the differential switching circuit and said second switch element being connected between said first node and a third node of the differential switching circuit; and the respective second nodes of the differential switching circuits of said plurality being connected together, and the respective third nodes of the differential switching circuits of said plurality being connected together; and the digital-to-analog converter further comprising a plurality of current sources or current sinks, corresponding respectively to the differential switching circuits of said plurality, each current source or current sink being connected operatively to said first node of its said corresponding differential switching circuit for causing a substantially constant current to flow therethrough when the converter is in use.